The ash content of coal has varied characteristics due to the diversification of coal sources of a fossil-fuel power plant, a conventional flue gas treatment device can only control the dust emission in a range of 20 to 100 mg/Nm3, however, with the increased attention on PM2.5 and increasingly strict requirement for environment protection, the emission standard of the fossil-fuel power plant is required to reach a range of 20 to 30 mg/Nm3, even an emission standard of 5 mg/Nm3, which is similar to the emission requirement for the natural gas furnace, is required in developed regions. A wet electrostatic precipitator, which is used in the chemical industry, is applied to the fossil-fuel power plant to meet the stricter requirement for dust emission.
A wet desulphurization technology can remove most of SO2, and the flue gas after the wet desulphurization is a nearly saturated wet flue gas, and generally has a temperature about 50 Celsius degree, and if such flue gas enters a stack directly, the following issues will be caused.
The temperature of about 50 Celsius degree is below several acid dew points of the flue gas. In the flue gas, a small amount of sulfur oxides, which are not completely removed after the flue gas passes through a desulfurization reaction tower, and chlorides and fluorides, which are not removed, can generate acids having a strong corrosivity in the presence of water, which causes serious corrosion to the subsequent flues and stacks, and threatens the safe running of the power plant. Thus, expensive metallic or non-metallic materials are generally used in the wet stack for corrosion prevention, which has a high requirement for cost and construction.
When a low temperature desulphurized flue gas directly leaves the stack, vapor in the flue gas mixes with the air and rapidly congeals to generate small water drops, thereby generating white smoke or light rain, that is, a water floating phenomenon, which causes visual pollution. Furthermore, since the flue gas contains solid matters such as dusts and gypsum in the desulphurization system and the low temperature flue gas after leaving the stack has a low rising height, the solid matters will be scattered around the stack in a small area, which causes a phenomenon of gypsum rain floating around the power plant, thereby seriously polluting the environment.
To address the above issues, the flue gas is passed through the wet electrostatic precipitator and a flue gas heater in sequence, and then is discharged into the stack. However, since the wet electrostatic precipitator and the flue gas heater are located at different positions and the devices are independent, a large area is required for arranging these devices.
Furthermore, each bidirectional transition flue can create a large pressure drop, and the flue gas has poor flow uniformity, thus a flow equalization orifice plate may be required which further increases the resistance, thereby causing a high pressure drop of the flue gas side, an increased pressure head of a draught fan, and an increased power consumption.
In conclusion, a technical issue to be solved by those skilled in the art is to reduce an occupied area of the device and the resistance for the flue gas.